Notifying bit allocation changes in a multicarrier modulation communications system

ABSTRACT

In a multicarrier modulation communications system, a change of a bit allocation table for the multiple subchannels or tones is notified from a transmitter to a receiver by a change in the inversion state of synchronization symbols provided in superframes of transmitted multicarrier symbols, successive changes of the bit allocation table being indicated by opposite changes of the inversion state of the synchronization symbols, respectively from non-inverted to inverted synchronization symbols and from inverted to non-inverted synchronization symbols. This provides resistance to errors and facilitates rapid recovery in the event that errors occur. A superframe number can also be used to identify when the inversion state of the synchronization symbols is to be changed.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/221,521 filed Jul. 28, 2000, the entire contents anddisclosure of which are hereby incorporated herein by reference.

[0002] This invention relates to notifying bit allocation changes in amulticarrier modulation communications system.

BACKGROUND OF THE INVENTION

[0003] So-called ADSL (asymmetric digital subscriber line)communications systems are known in the form of multicarrier modulation(MCM) communications systems which are conveniently implemented asdiscrete multitone (DMT) systems using Discrete Fourier Transform (DFT)and inverse DFT (IDFT) techniques. American National Standard forTelecommunications document T1.413 describes a particular form of suchan ADSL system, and the invention is accordingly described below in thecontext of such an ADSL system. However, it can be appreciated that theinvention is applicable to other forms of MCM communications systems,whether or not they are DMT systems and whether or not they provideasymmetric communications.

[0004] A T1.413 ADSL system uses a superframe structure in which eachsuperframe is composed of 68 data frames, numbered 0 to 67, which areencoded and modulated into DMT symbols, followed by a synchronizationsymbol (also referred to as a synchronization frame), which is insertedby the modulator to establish superframe boundaries. From a user dataperspective the DMT symbol period is 0.25 ms corresponding to a DMTsymbol rate of 4000 symbols per second, but in order to allow for theinsertion of the synchronization symbol the transmitted DMT symbol rateis increased by a factor of 69/68; the superframe period is 17 ms.

[0005] In each data frame, buffered data is allocated to the differenttone or carrier subchannels in a manner which is dependent upon thesignal-to-noise ratios (SNRs) of the subchannels, typically so that biterror rates (BERs) of the subchannels, as monitored at the receiver, aresubstantially equal. As a result, within each data frame differentsub-channels carry different numbers of bits. With an appropriateallocation of bits and transmit powers to the different subchannels,such a system can provide a desirable performance.

[0006] Over a period of time during normal operation of such an ADSLsystem, for example with changes in temperature, traffic on adjacenttransmission channels, and other sources of interference, the SNRs ofthe different subchannels will vary, so that it is desirable to updatethe allocations of bits to subchannels during operation of the system.Peter S. Chow et al. U.S. Pat. No. 5,479,447 issued Dec. 26, 1995,entitled “Method And Apparatus For Adaptive, Variable Bandwidth,High-Speed Data Transmission Of A Multicarrier Signal Over DigitalSubscriber Lines”, discloses a procedure for initializing bitallocations in an MCM system, and proposes an adaptive updatingprocedure for such allocations.

[0007] It can be appreciated that the bit allocations which are used bythe transmitter and the receiver must be matched, and that changes ofthese bit allocations must be made at the same time (i.e. for the samesuperframe) in the transmitter and the receiver in a manner which is notsubject to errors. Otherwise, the BER can be seriously impaired, andthere may be a resulting need to fully re-initialize the system: a veryundesirable consequence, especially considering the time (of the orderof 10 seconds) required for full initialization.

[0008] To avoid errors in the bit allocation updating procedure,referred to briefly as bit swapping but more accurately relating to anupdating of bit allocations and signal energies, or transmit gains, inwhat is referred to as a B&G (bits and gains) table maintained in eachof the transmitter and the receiver) bit swap request messages can besent from the receiver to the transmitter using an ADSL overhead controlchannel (referred to as the aoc) which is part of the ADSL framingstructure. Each aoc message is transmitted 5 consecutive times, toprovide a high degree of reliability, and the message can also besubject to forward error correction coding (FECC), codewordinterleaving, and trellis coding of the system. In response to the bitswap message, the transmitter can send an acknowledge message on theaoc, and the bit allocation is subsequently performed at both thetransmitter and the receiver.

[0009] Ronald R. Hunt et al. U.S. Pat. No. 5,400,322 issued Mar. 21,1995, entitled “Updating Of Bit Allocations In A Multicarrier ModulationTransmission System”, discloses such a so-called bit swapping procedure,together with a numbering of DMT symbols for synchronizing bitallocation changes between the transmitter and receiver of an ADSLsystem. In addition, this patent recognizes that because the subchannelscarry variable numbers of bits, the total transmission rate of thesystem is not fixed but can be increased (resulting in reduced SNR) ordecreased (resulting in increased SNR) to meet particular requirements.

[0010] Such variation in transmission rate, now referred to as seamlessrate adaptation (SRA), serves to adapt the data transmission rate duringoperation of the ADSL system in a so-called seamless manner, i.e.without errors in or interruption of the data transmission. As can beappreciated from the above description, SRA involves changing the numberof bits modulated in a DMT symbol (i.e. one data frame), and changingthe B&G tables accordingly, without modifying other parameters of theADSL system such as the FECC, interleaving, and framing. It can beappreciated that error-free changes to the B&G tables in a manner thatis synchronized between the transmitter and the receiver is as importantfor SRA as it is for conventional bit swapping, for similar reasons.

[0011] Whereas conventional bit swapping is initiated by the receiver,SRA can be initiated by either the receiver or the transmitter. In theformer case the receiver sends an SRA request message to thetransmitter, and in the latter case the transmitter sends an initial SRArequest message to the receiver which can accept this request by sendingan SRA request message to the transmitter. In each case the transmittercan grant the request by sending a signal referred to as SRA_GO to thereceiver, after which the transmitter and receiver both use changed B&Gtables accordingly.

[0012] In either conventional bit swapping or SRA, there remains a riskof errors causing a loss of synchronism between the B&G tables used inthe transmitter and the receiver. For example, the acknowledge messageor SRA_GO signal may be transmitted by the transmitter but may becomecorrupted due to a burst of errors and not recognized by the receiver.In this case, the transmitter will switch to a changed B&G table but thereceiver will not, resulting in such a loss of synchronism.

[0013] In order to address such a problem, “The Essential Merit OfBit-Swapping” by John M. Cioffi suggests that after transmitting a bitswap request message the receiver can monitor the incoming signal fromthe transmitter using both old and new B&G tables, and can use an FECCerror flag to determine which of the tables is correct and shouldcontinue to be used. This proposal would be relatively inconvenient andcostly to implement, because the FECC operates on data bytes rather thanon the subchannels or tones and their allocated bits so that translationbetween these would be necessary, and is further complicated byinterleaving.

[0014] It has been proposed in “Proposed working text for Seamless RateAdaptation (SRA) for G.dmt.bis and G.lite.bis”, Aware, Inc.,ITU—Telecommunication Standardization Sector, Study Group 15, TemporaryDocument BA-087, 19-23 June 2000, to implement the SRA GO signal as aninverted synchronization symbol (180 degree phase shift). It will berecalled from the description above that the synchronization symbol,which comprises a known pseudo-random data sequence or pattern, isinserted after every 68 data frames (DMT symbols) to establishsuperframe boundaries.

[0015] It is observed here that during initialization of a T1.413 ADSLsystem, various signals are used including signals which can begenerically referred to as REVERB and SEGUE. The REVERB signals use thesame pseudo-random data pattern as is used by the synchronizationsymbols. Except for a pilot tone, the SEGUE signals comprise atone-by-tone 180 degree phase reversal of the REVERB signals (i.e. +mapsto -, and −maps to +, for each signal point of the 4-QAM constellationthat is used). The pilot tone is a specific subchannel or tone ontowhich the data modulated is a constant {0,0}, generating the {+,+} 4-QAMconstellation point, used to facilitate resolution of sample timing.Thus except for this pilot tone, the SEGUE signal is an inversion of theREVERB signal, corresponding to the synchronization symbol. As usedherein inversion of the synchronization symbol includes such atone-by-tone 180 degree phase reversal for all or substantially all ofthe tones, whether or not this includes a 180 degree phase reversal forany specific tones used for special purposes, such as the pilot tone.Terms such as inversion and inverted as used herein are to be understoodaccordingly.

[0016] However, it has been recognized that with the above proposal aburst of noise might conceivably cause a normal synchronization symbolto be interpreted as an inverted synchronization symbol, in which casethe receiver could prematurely implement a change in its B&G table. Toreduce this risk, it has been proposed in “G.gen: SRA features andmessages”, Alcatel, ITU—Telecommunication Standardization Sector, StudyGroup 15, Temporary Document HC-049, Jul. 31-Aug. 4, 2000, to send asuperframe count with the SWA swap request message to identify where theSRA_GO signal is expected, and for the receiver to monitor only theparticular expected synchronization symbol for inversion to constitutethe SRA_GO signal. Even in this case, there is a possibility that theactually inverted synchronization symbol constituting the SRA_GO signalmay be corrupted by noise so that it is wrongly interpreted as anon-inverted synchronization symbol, in which case again the transmitterwould implement the relevant SRA changes and the receiver would not.

[0017] In the various error situations discussed above, recoverytechniques can be used for example as described by John M. Cioffi asdiscussed above, or re-initialization or retraining of the system, butthese techniques are not particularly desirable because of their adverseimpacts on the architecture and complexity of the system and/or datatransmission.

[0018] A need exists, therefore, to provide an improved method ofnotifying a receiver of bit allocation changes in a multicarriermodulation communications system.

SUMMARY OF THE INVENTION

[0019] According to one aspect, this invention provides a method ofnotifying a receiver of a bit allocation change in a multicarriermodulation communications system in which data frames andsynchronization frames are transmitted in superframes from a transmitterto the receiver, comprising the steps of: at the transmitter, changingan inversion state of successive synchronization frames to notify thereceiver of a bit allocation change; and at the receiver, detecting achange in inversion state of received synchronization frames todetermine the bit allocation change.

[0020] The invention also provides a transmitter for a multicarriermodulation communications system for use in carrying out the methodrecited above, the transmitter comprising: information for allocatingdata bits to multicarrier subchannels for transmission in respectivedata frames; a control unit for controlling transmission of the dataframes in superframes each including a synchronization frame; and meansfor changing an inversion state of the synchronization frames toindicate a change of said information, successive changes of saidinformation being indicated by opposite changes of the inversion stateof the synchronization frames, respectively from non-inverted toinverted synchronization frames and from inverted to non-invertedsynchronization frames.

[0021] The invention further provides a receiver for a multicarriermodulation communications system for use in carrying out the methodrecited above, the receiver comprising: information for decodingreceived multicarrier subchannel data frames to data bits in accordancewith bit allocations, the data frames being received in superframes eachincluding a synchronization frame; and a control unit for detecting achange of an inversion state of successive synchronization frames andchanging said bit allocations in response to such detection, successivechanges of said bit allocations being effected in response to detectionof opposite changes of the inversion state of the synchronizationframes, respectively from non-inverted to inverted synchronizationframes and from inverted to non-inverted synchronization frames.

[0022] According to another aspect, this invention provides a method ofnotifying a receiver of bit allocation changes in a DMT (discretemultitone) communications system in which information is transmittedfrom a transmitter to the receiver in superframes each of whichcomprises a plurality of DMT data symbols and a synchronization symbol,comprising the steps of: at the transmitter, changing an inversion stateof the synchronization symbols, from non-inverted to inverted and frominverted to non-inverted, to notify the receiver of successive bitallocation changes; and at the receiver, detecting changes in theinversion state of received synchronization symbols, from non-invertedto inverted and from inverted to non-inverted, to determine thesuccessive bit allocation changes.

[0023] The invention further provides a transmitter for a DMT (discretemultitone) communications system for use in carrying out the abovemethod, the transmitter comprising: a bit allocation table in accordancewith which data bits are allocated to tones for transmission inrespective DMT symbols; a control unit for controlling transmission ofthe DMT symbols in superframes each including a synchronization symbol;and means for changing an inversion state of the synchronization symbolsto indicate a change of the bit allocation table, successive changes ofthe bit allocation table being indicated by opposite changes of theinversion state of the synchronization symbols, respectively fromnon-inverted to inverted synchronization symbols and from inverted tonon-inverted synchronization symbols.

[0024] The means for changing an inversion state of the synchronizationsymbols to indicate a change of the bit allocation table can comprisemeans for changing a sign of inputs to or outputs from an IDFT (InverseDiscrete Fourier Transform) of the transmitter.

[0025] Alternatively, the means for changing an inversion state of thesynchronization symbols to indicate a change of the bit allocation tablecan comprise means for selectively inverting an output of a pseudorandom data source in accordance with which tones of the synchronizationsymbols are modulated.

[0026] The invention also provides a receiver for a DMT (discretemultitone) communications system for use in carrying out the abovemethod, the receiver comprising: a bit allocation table in accordancewith which DMT symbols are decoded to data bits, the DMT symbols beingreceived in superframes each including a synchronization symbol; and acontrol unit for detecting a change of an inversion state of successivesynchronization symbols and changing the bit allocation table inresponse to such detection, successive changes of the bit allocationtable being effected in response to detection of opposite changes of theinversion state of the synchronization symbols, respectively fromnon-inverted to inverted synchronization symbols and from inverted tonon-inverted synchronization symbols.

[0027] According to a further aspect, the invention provides a method ofeffecting bit allocation changes in a DMT (discrete multitone)communications system in which information is transmitted from atransmitter to a receiver in superframes each of which comprises aplurality of DMT data symbols and a synchronization symbol, comprisingthe steps of: at the transmitter, in response to a request from thereceiver for a change of bit allocations, sending to the receiver anacknowledgement identifying a superframe and, commencing with theidentified superframe, changing an inversion state of thesynchronization symbols, from non-inverted to inverted or from invertedto non-inverted, and implementing said bit allocation changes; and atthe receiver, sending said request for a change in bit allocations,detecting the acknowledgement and identified superframe, and detectingthe change in the inversion state of received synchronization symbolsand implementing said bit allocation changes commencing with theidentified superframe.

[0028] Conveniently this method comprises the steps of counting thesuperframes in synchronism at the transmitter and at the receiver, theacknowledgement identifying a superframe comprising a superframe number.

[0029] The invention also provides a transmitter for a DMT (discretemultitone) communications system for use in carrying out this method,comprising: a bit allocation table in accordance with which data bitsare allocated to tones for transmission in respective DMT symbols; acontrol unit for controlling transmission of the DMT symbols insuperframes each including a synchronization symbol, the control unitincluding means for sending said acknowledgement identifying asuperframe in response to a request from the receiver for a change ofbit allocations; and means for changing said bit allocation table and aninversion state of the synchronization symbols commencing with theidentified superframe.

[0030] The invention further provides a receiver for a DMT (discretemultitone) communications system for use in carrying out this method,the receiver comprising: a bit allocation table in accordance with whichDMT symbols are decoded to data bits, the DMT symbols being received insuperframes each including a synchronization symbol; and a control unitfor sending said request for a change in bit allocations, detecting theacknowledgement and identified superframe, and detecting the change inthe inversion state of received synchronization symbols and changingsaid bit allocation table commencing with the identified superframe.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention will be further understood from the followingdescription with reference to the accompanying drawings, in which by wayof example:

[0032]FIG. 1 illustrates a functional block diagram of a transmitter anda receiver of an ADSL communications system using DMT modulation;

[0033]FIG. 2 is a flow chart with reference to which operations in thetransmitter and receiver in accordance with one embodiment of theinvention are described; and

[0034]FIG. 3 is a flow chart illustrating operations in the transmitterand receiver in accordance with another embodiment of the invention.

DETAILED DESCRIPTION

[0035] Referring to the drawings, FIG. 1 illustrates functional blocksof a transmitter 10 and a receiver 20 of an ADSL communications system,which generally has the known form of a T1.413 ADSL system. As is known,the transmitter 10 comprises data multiplexing, CRC (cyclic redundancycheck, scrambling and FECC (forward error correction coding),interleaving, and tone ordering functions represented by a unit 12;constellation encoding and gain scaling functions represented by a unit14; IDFT (Inverse DFT), data buffering and parallel to serialconversion, D-A (digital to analog) conversion, and data transmissionfunctions represented by a unit 16; and a transmitter control unit 18.The receiver 20 has converse functions, including receiving, filtering,equalization, A-D (analog to digital) conversion, data buffering andserial to parallel conversion, and DFT (Discrete Fourier Transform)functions represented by a unit 22; a decoder 24; data buffering anddeinterleaving, FECC decoding, and data demultiplexing functionsrepresented by a unit 26; and a receiver controller 28.

[0036] Signals transmitted from the transmitter 10 to the receiver 20are carried via a path 30 between the units 16 and 22 represented by adashed line in FIG. 1 and for example constituted by a two-wiretelephone line. Although not shown in FIG. 1 for clarity, the ADSLsystem transmits signals in both directions on the path 30, andaccordingly the system also includes similar transmit functions at thereceiver 20 end of the path 30, and corresponding receiver functions atthe transmitter 10 end of the path 30, in known manner.

[0037] The transmitter 10 and receiver 20 of the ADSL system of FIG. 1operate in the generally known manner of a T1.413 ADSL system which neednot be described here. Briefly, data to be transmitted by thetransmitter 10, and the overhead control channel or aoc supplied by thetransmitter control unit 18 on a line 32, are supplied to the unit 12,which for each data frame or DMT symbol provides a set of bits to theunit 14. The unit 14 allocates these bits to respective tones(multicarrier subchannels) and provides respective signal energies ortransmit gains in accordance with a B&G (bits and gains) table 34 storedin the transmitter control unit 18, and the resulting encodedinformation is transmitted to the path 30 via the functions of the unit16. After every 68 data frames or DMT symbols, the transmitter inserts asynchronization symbol (also referred to as a sync frame) which carriesa pseudo random data (PRD) pattern supplied by a PRD source 36 in thetransmitter control unit 18. Control paths are accordingly provided andillustrated in FIG. 1 between the transmitter control unit 18 and theunits 12, 14, and 16. A further path 38 is also provided, from thetransmitter control unit 18 to the unit 16 as shown in FIG. 1, asdescribed further below.

[0038] Conversely, the signals received by the receiver 20 via the path30 are processed by the unit 22 and decoded by the decoder 24 inaccordance with a corresponding B&G table 44 stored in the receivercontrol unit 28, the decoded data being supplied to the unit 26 toprovide received data and the aoc, the latter being supplied by thetransmitter control unit 18 on a line 32, are supplied to the receivercontrol unit 28 via a line 42. The receiver control unit 28 alsoincludes a corresponding PRD source 46 for use in recognizing eachsynchronization symbol. Control paths are provided and illustrated inFIG. 1 between the receiver control unit 28 and the units 22, 24, and26. A further path 48 is also provided, from the unit 22 to the receivercontrol unit 28 as shown in FIG. 1, as described further below.

[0039] As described above, it is necessary to ensure that the B&G tables34 and 44 in the transmitter 10 and the receiver 20 contain the sameinformation, and to allow this information to be changed or updated toaccommodate changes in SNR which occur over time, and/or for SRA, i.e.different numbers of data bits being allocated to different toneswithout interruption or disturbance of the data transmission. The aocprovides a communication path from the transmitter control unit 18 tothe receiver control unit 28, and a corresponding aoc path is providedin the opposite direction of transmission by the corresponding units,not shown in FIG. 1, but mentioned above as being provided for theopposite direction of transmission. These aoc paths are used in areliable manner (for example, using redundant transmissions of eachmessage) for carrying information for updating the B&G tables.

[0040] However, as discussed above it remains necessary to provide areliable notification of the changes in the B&G tables 34 and 44, sothat changes in the encoding and decoding of the DMT symbols are alwaysmatched between the transmitter 10 and the receiver 20. Suchnotification is desired to be very resistant to errors due, for example,to noise bursts. Furthermore, in the event of such an error, it isdesirable for the ADSL system to recover in the shortest possible time,without requiring retraining or re-initialization of the system. Thislast point is not particularly addressed by the various proposalsdiscussed above. Further, it is desirable for these objectives to be metin a simple and convenient manner, without requiring any significanthardware changes or relatively complex processing of signals.

[0041] As described above, it has been proposed to invert thesynchronization symbol to provide a notification of when the changed orupdated B&G tables are to start being used, and, for robustness, for thereceiver to monitor for inversion only the one synchronization symbolwhich it expects to be inverted in response to a request message that ithas previously sent.

[0042] The present invention differs from this in that, rather thaninverting one synchronization symbol to provide a notification, itinverts not only the current synchronization symbol but also eachsubsequent synchronization symbol. This inversion of all of thesynchronization symbols continues until there is a subsequent change orupdate of the B&G tables, for the notification of which the inversionstate of each of the current and subsequent synchronization symbols isagain changed, so that these synchronization symbols are transmitted intheir non-inverted form. This alternation of sequences of non-invertedand inverted synchronization symbols continues in a similar manner, withthe inversion state of all of the subsequent synchronization symbolsbeing changed to represent each notification of a switch to changed orupdated B&G tables.

[0043] This seemingly simple change provides a radical improvement overthe prior proposals discussed above. As in those proposals, it isconceivable that due to a burst of errors the inversion state of asynchronization symbol may be wrongly detected by the receiver.Consequently, if the receiver has previously sent a request message foran update of the B&G tables so that it is expecting a change in theinversion state of the synchronization symbol, and due to errors iteither detects prematurely a change in the inversion state of asynchronization symbol, or fails to detect a change in the inversionstate of a synchronization symbol, then this error condition willpersist for only one superframe, because the receiver will detect theerror from the inversion state of the next synchronization symbol. Theprobability of the inversion state of two successive synchronizationsymbols both being detected incorrectly is very small indeed, and eventhis error situation would be detected after another superframe. On theother hand, if the receiver detects a change in the inversion state of asynchronization symbol when it has not previously sent a request messagefor an update of the B&G tables so that no such change in inversionstate is expected, it can immediately determine that an error hasoccurred.

[0044] Thus the receiver can quickly, i.e. typically after at most onesuperframe even when there has been an error in the detection of theinversion state of the synchronization symbol, detect and correct anerror so that the correct B&G tables are always used. In this respect itcan be seen that typically at most one superframe of data could bedecoded in the receiver using the wrong B&G table, either prematurelyupdated or not updated when it should have been. A recovery from such anerror situation can be carried out by reprocessing the superframe ofdata using the correct B&G table as soon as the error is detected,and/or in other known ways up to and including retransmission of thedata. In any event, there is no resulting need for either a longrecovery from the error condition or retraining or re-initialization ofthe ADSL system, so that the disadvantages associated with theseprocesses are avoided.

[0045] From the above description it can be appreciated that a change inthe inversion state of the synchronization symbols means either a changefrom non-inverted synchronization symbols to inverted synchronizationsymbols, or a change from inverted synchronization symbols tonon-inverted synchronization symbols.

[0046] There are various ways in which the transmitter 10 can effect theinversion of each synchronization symbol which is to be inverted. Forexample, the transmitter control unit 18 can, as represented in FIG. 1by the line 38, provide a signal to the unit 16 which produces aninversion of the sign of all of the inputs to the IDFT. Alternatively,such a signal supplied via the line 38 to the unit 16 can produce aninversion of the sign of the samples produced at the outputs of theIDFT. As a further alternative, the transmitter control unit 18 canselectively invert the output of the PRD source 36 which is used toprovide data for the synchronization symbols. In the receiver 20,converse measures are taken to enable detection of the inversion stateof each synchronization symbol, this information being supplied to thereceiver control unit from the unit 22 via the line 48.

[0047]FIG. 2 is a flow chart which represents, in a simplified manner tofacilitate illustration and understanding, steps carried out by thetransmitter 10 and the receiver 20 in a notification process asdescribed above. The left-hand side of FIG. 2 represents steps carriedout at the transmitter 10, and the right-hand side of FIG. 2 representssteps carried out at the receiver 20. Bold arrowed lines 50 and 54inclined downwardly to the right represent signals transmitted from thetransmitter 10 to the receiver 20; a bold arrowed line 52 inclineddownwardly to the left represents signal transmission from the receiver20 to the transmitter 10, using the aoc in the opposite direction oftransmission as described above.

[0048]FIG. 2 represents steps related to SRA initiated at thetransmitter 10, and it is described below how this differs for SRAinitiated at the receiver 20. It can be appreciated that similar stepsare desirably carried out for a bit swap process initiated at thereceiver 20 as is conventional, or for any other situation in which itmay be desired to provide notification of a switch to changed or updatedB&G tables.

[0049] Referring to FIG. 2, an SRA is initiated at the transmitter 10 asshown by a block 60 by the transmitter control unit 18 sending a requestmessage SRA-T via the aoc, as shown by the line 50. As shown by a block70, the receiver control unit 28 determines whether an SRA-T requestmessage has been received via the aoc, if not takes no action as shownby a block 71, and if so determines at a block 72 whether or not toaccept this request. If the receiver control unit 28 does not accept theSRA request, no further action is taken as indicated by a block 73. Ifthe receiver control unit 28 accepts the SRA request, at a block 74 ittransmits, via the aoc in the opposite direction as indicated by theline 52, a request message SRA-R. In the transmitter 10, after sendingthe SRA-T request message at the block 60 (and with an appropriatewaiting period which is not separately shown in FIG. 2) the transmittercontrol unit 18 determines at a block 61 whether an SRA-R requestmessage has been received within a timeout period; if not it can returnto the block 60 to retransmit the SRA-T message.

[0050] For SRA initiation at the receiver 20, the steps 70 to 73 at thereceiver 20 are omitted, the SRA initiation starting with the receivercontrol unit 28 sending the SRA-R request message at the block 74.Correspondingly, at the transmitter 10 the step 60 is omitted, thetransmitter control unit 18 simply determining at the block 61 whetheror not a request message SRA-R has been received from the receiver 20.If not, in this case no action is taken at the transmitter 10.Accordingly, for SRA initiation at the receiver 20 the steps omit theblocks shown above a horizontal line A-A in FIG. 2.

[0051] In response to a request message SRA-R received at the block 61,the transmitter control unit 18 determines at a block 62 whether or notto accept this request. If the transmitter control unit 18 does notaccept the SRA request, no further action is taken as indicated by ablock 63. If the transmitter control unit 18 accepts the SRA request, itsubsequently at a block 64 changes the inversion state of thesynchronization symbols that it transmits, as described above. Thus ifeach synchronization symbol has previously been transmitted innon-inverted form, then the transmitter starts to transmit thesuccessive synchronization symbols in inverted form, and if eachsynchronization symbol has previously been transmitted in inverted form,then the transmitter starts to transmit the successive synchronizationsymbols in non-inverted form. As shown by a block 65, with this changein inversion state of the synchronization symbols the transmittercontrol unit starts to use the new or updated B&G table 34 for theoperation of the unit 14.

[0052] Having sent the request message SRA-R at the block 74, thereceiver control unit 28 expects a change in the inversion state of thesynchronization symbols, which it determines at a block 75 (with anappropriate waiting period which is not separately shown in FIG. 2). Inthe absence of such a change in the inversion state of thesynchronization symbols being detected within a timeout period, thereceiver can conclude that an error has occurred and can return to theblock 74 to retransmit the SRA-R request message. On detection of theexpected change in the inversion state of the synchronization symbols,as shown by a block 76 the receiver control unit 28 starts to use thenew or updated B&G table 44 for the operation of the decoder 24. In theevent of a detected error, the receiver control unit 28 can initiaterapid recovery measures for example as outlined above.

[0053]FIG. 3 is a flow chart which represents, in a simplified mannersimilar manner to FIG. 2 and using the same references whereappropriate, steps carried out by the transmitter 10 and the receiver 20in a notification process in accordance with another embodiment of theinvention, in this case initiated at the receiver 20 with the receivercontrol unit 28 sending the SRA-R request message at the block 74. Atthe transmitter 10, the transmitter control unit 18 determines at theblock 61 whether or not a request message SRA-R has been received fromthe receiver 20. If not, no action is taken at the transmitter 10, asshown by a block 80. The transmitter control unit 18 again determines atthe block 62 whether or not to accept the SRA request, and accordinglyeither takes no further action as indicated by the block 63 or proceedsto a block 81.

[0054] In the latter case, as shown in FIG. 3 by the block 81 thetransmitter control unit 18 determines a number of a future superframefor which it will introduce a change in the inversion state of thesynchronization symbols, and sends to the receiver control unit 28, viathe aoc as indicated by a line 56, an acknowledgement (ACK) of thereceived and accepted SRA-R request message with this superframe number.To this end, the superframes are counted in synchronism at thetransmitter 10 and the receiver 20, as is already known in the art andmay be carried out for other purposes. The transmitter control unit 18then waits, as indicated by a block 82, for this superframe number, andfor this and subsequent superframes changes the inversion state of thesynchronization symbols and uses the new B&G table, as represented inFIG. 3 by the blocks 64 and 65 and as already described above.

[0055] In the receiver 20, at a block 90 (with an appropriate waitingperiod which is not separately shown in FIG. 3) the receiver controlunit 28 detects the acknowledgement of its SRA-R request message withthe superframe number determined at the transmitter 10, and at a block91 waits for the occurrence of this numbered superframe. It then, at ablock 92, checks for the expected change in the inversion state of thesynchronization symbol, and as shown by the block 76 then starts to usethe new or updated B&G table 44 for the operation of the decoder 24. Ifthe acknowledgement is not received at the block 90 within a timeoutperiod, or if the expected change in the inversion state of thesynchronization symbol of the numbered superframe is not detected at theblock 92, the receiver control unit 28 concludes that an error hasoccurred and proceeds to an error recovery process represented by ablock 93. The error recovery process can include steps for example asdescribed above for rapid recovery from the detected error.

[0056] Although particular embodiments of the invention are described indetail above, it can be appreciated that numerous modifications,variations, and adaptations may be made within the scope of theinvention as defined in the claims.

What is claimed is:
 1. A method of notifying a receiver of a bitallocation change in a multicarrier modulation communications system inwhich data frames and synchronization frames are transmitted insuperframes from a transmitter to the receiver, comprising the steps of:at the transmitter, changing an inversion state of successivesynchronization frames to notify the receiver of a bit allocationchange; and at the receiver, detecting a change in inversion state ofreceived synchronization frames to determine the bit allocation change.2. A transmitter for a multicarrier modulation communications system foruse in carrying out the method of claim 1, the transmitter comprising:information for allocating data bits to multicarrier subchannels fortransmission in respective data frames; a control unit for controllingtransmission of the data frames in superframes each including asynchronization frame; and means for changing an inversion state of thesynchronization frames to indicate a change of said information,successive changes of said information being indicated by oppositechanges of the inversion state of the synchronization frames,respectively from non-inverted to inverted synchronization frames andfrom inverted to non-inverted synchronization frames.
 3. A receiver fora multicarrier modulation communications system for use in carrying outthe method of claim 1, the receiver comprising: information for decodingreceived multicarrier subchannel data frames to data bits in accordancewith bit allocations, the data frames being received in superframes eachincluding a synchronization frame; and a control unit for detecting achange of an inversion state of successive synchronization frames andchanging said bit allocations in response to such detection, successivechanges of said bit allocations being effected in response to detectionof opposite changes of the inversion state of the synchronizationframes, respectively from non-inverted to inverted synchronizationframes and from inverted to non-inverted synchronization frames.
 4. Amethod of notifying a receiver of bit allocation changes in a DMT(discrete multitone) communications system in which information istransmitted from a transmitter to the receiver in superframes each ofwhich comprises a plurality of DMT data symbols and a synchronizationsymbol, comprising the steps of: at the transmitter, changing aninversion state of the synchronization symbols, from non-inverted toinverted and from inverted to non-inverted, to notify the receiver ofsuccessive bit allocation changes; and at the receiver, detectingchanges in the inversion state of received synchronization symbols, fromnon-inverted to inverted and from inverted to non-inverted, to determinethe successive bit allocation changes.
 5. A transmitter for a DMT(discrete multitone) communications system for use in carrying out themethod of claim 4, the transmitter comprising: a bit allocation table inaccordance with which data bits are allocated to tones for transmissionin respective DMT symbols; a control unit for controlling transmissionof the DMT symbols in superframes each including a synchronizationsymbol; and means for changing an inversion state of the synchronizationsymbols to indicate a change of the bit allocation table, successivechanges of the bit allocation table being indicated by opposite changesof the inversion state of the synchronization symbols, respectively fromnon-inverted to inverted synchronization symbols and from inverted tonon-inverted synchronization symbols.
 6. A transmitter as claimed inclaim 5 wherein the transmitter includes an IDFT (Inverse DiscreteFourier Transform) and the means for changing an inversion state of thesynchronization symbols to indicate a change of the bit allocation tablecomprises means for changing a sign of inputs to or outputs from theIDFT.
 7. A transmitter as claimed in claim 5 wherein the transmitterincludes a pseudo random data source in accordance with which tones ofthe synchronization symbols are modulated, and the means for changing aninversion state of the synchronization symbols to indicate a change ofthe bit allocation table comprises means for selectively inverting anoutput of the pseudo random data source.
 8. A receiver for a DMT(discrete multitone) communications system for use in carrying out themethod of claim 4, the receiver comprising: a bit allocation table inaccordance with which DMT symbols are decoded to data bits, the DMTsymbols being received in superframes each including a synchronizationsymbol; and a control unit for detecting a change of an inversion stateof successive synchronization symbols and changing the bit allocationtable in response to such detection, successive changes of the bitallocation table being effected in response to detection of oppositechanges of the inversion state of the synchronization symbols,respectively from non-inverted to inverted synchronization symbols andfrom inverted to non-inverted synchronization symbols.
 9. A method ofeffecting bit allocation changes in a DMT (discrete multitone)communications system in which information is transmitted from atransmitter to a receiver in superframes each of which comprises aplurality of DMT data symbols and a synchronization symbol, comprisingthe steps of: at the transmitter, in response to a request from thereceiver for a change of bit allocations, sending to the receiver anacknowledgement identifying a superframe and, commencing with theidentified superframe, changing an inversion state of thesynchronization symbols, from non-inverted to inverted or from invertedto non-inverted, and implementing said bit allocation changes; and atthe receiver, sending said request for a change in bit allocations,detecting the acknowledgement and identified superframe, and detectingthe change in the inversion state of received synchronization symbolsand implementing said bit allocation changes commencing with theidentified superframe.
 10. A method as claimed in claim 9 and comprisingthe steps of counting the superframes in synchronism at the transmitterand at the receiver, wherein the acknowledgement identifying asuperframe comprises a superframe number.
 11. A transmitter for a DMT(discrete multitone) communications system for use in carrying out themethod of claim 10, the transmitter comprising: a bit allocation tablein accordance with which data bits are allocated to tones fortransmission in respective DMT symbols; a control unit for controllingtransmission of the DMT symbols in superframes each including asynchronization symbol, the control unit including means for sendingsaid acknowledgement identifying a superframe in response to a requestfrom the receiver for a change of bit allocations; and means forchanging said bit allocation table and an inversion state of thesynchronization symbols commencing with the identified superframe.
 12. Atransmitter as claimed in claim 11 wherein the transmitter includes anIDFT (Inverse Discrete Fourier Transform) and the means for changing aninversion state of the synchronization symbols comprises means forchanging a sign of inputs to or outputs from the IDFT.
 13. A transmitteras claimed in claim 11 wherein the transmitter includes a pseudo randomdata source in accordance with which tones of the synchronizationsymbols are modulated, and the means for changing an inversion state ofthe synchronization symbols comprises means for selectively inverting anoutput of the pseudo random data source.
 14. A receiver for a DMT(discrete multitone) communications system for use in carrying out themethod of claim 10, the receiver comprising: a bit allocation table inaccordance with which DMT symbols are decoded to data bits, the DMTsymbols being received in superframes each including a synchronizationsymbol; and a control unit for sending said request for a change in bitallocations, detecting the acknowledgement and identified superframe,and detecting the change in the inversion state of receivedsynchronization symbols and changing said bit allocation tablecommencing with the identified superframe.